Hydrogen (H2) gas has many uses. For example, it is the main propellant in spaceships and commercial and military launch vehicles. It is also used extensively in scientific research and industry, notably in the manufacturing of glass and steel as well as in the refining of petroleum products.1 In 2003 the U.S. Department of Energy accelerated its hydrogen program to develop the technology needed for commercially viable hydrogen-powered fuel cells—a way to power cars, trucks, homes, and businesses that could significantly reduce pollution and greenhouse gas emissions as well as our dependence on fossil fuels.2 However, H2 gas is highly volatile and, when in contact with oxygen, can become extremely flammable and highly explosive. The use of effective H2 sensors to accurately and quickly respond to H2 gas leaks and to monitor manufacturing and distribution will be crucial for the safe deployment of all H2-based applications. For example, H2 gas detection in commercial and military launch vehicles is a great concern at both the propellant filling ground-support station and within the common booster core during ground operations.3 Fuel cells4 at the core of hydrogen-powered cars require two types of H2 sensors: sensors to monitor the quality of the hydrogen feed gas and, more importantly, sensors to detect leaks. These H2 sensors must be sensitive enough to discriminate between ambient low-level traces of hydrogen and those that are generated by a H2 leak.5 
A crucial parameter of H2 sensors in many applications is the response time. For example, the sensors that analyze H2 content in a mixed gas and monitor the reaction process require extremely short response times to follow the fuel cell's power generation and to shut down the engine in the event of a tank rupture. Currently, commercial sensors suffer from longer response times than the duty cycles likely needed for most applications.5,6 
Pd based H2 sensors have a unique advantage in that the surface of palladium can act catalytically to break the H—H bond in diatomic hydrogen, allowing monatomic hydrogen to diffuse into the material. Furthermore, palladium can dissolve more than 600 times its own volume of hydrogen, but dissolves little of the other common gases such as nitrogen, oxygen, nitrogen monoxide, carbon dioxide, and carbon monoxide. This allows Pd to be the most selective H2 sensing material.6 Finally, the Pd hydrogenation process is reversible at room temperature, enabling simpler designs and allowing for less power consumption by avoiding heating to achieve elevated temperatures.
In the presence of H2 the resistance of Pd will change due to the formation of a solid solution of Pd/H (at low H2 pressure, α-phase) or a hydride (at high H2 pressure, β-phase). The level of dissolved hydrogen changes the electrical resistivity of the metal and also its volume due to the formation of metal hydride. Thus, Pd is highly selective to H2, enabling Pd to be an excellent H2 sensing material. In fact, most of the room-temperature solid-state H2 sensors in a chemically variable environment use Pd metal and alloys as sensing elements.
Several fundamental problems are associated with bulk Pd-based hydrogen sensors. First, the diffusion of the hydrogen into bulk Pd such as a thick Pd film can result in an extraordinary large internal stress, leading to buckling of the films. This irreversible deformation leads to an irreversible resistance change. Secondly, the hydrogen atom diffusion in Pd is very slow at room temperature (the diffusion coefficient is 3.8×10−7 cm2/s at 298 K). Thus, the long diffusion pathway of hydrogen into bulk Pd structures inevitably results in a long response time.
Intensive research has been conducted in recent years to develop a new generation of H2 sensors with high speed, high sensitivity, miniature size and low cost.6-36 Nanomaterials7-31 have been a major focus in the search for high performance H2 sensing elements due to their large surface area to volume (SA/V) ratios which could enhance the absorption/desorption rates of a chemical reaction and allow for shorter H2 diffusion paths as well as confinement induced new properties. Among the various nanomaterials available, palladium (Pd) nanostructures7,8,13,18,22-24,26,27 have shown very promising properties suitable for fast H2 sensors.
Continuous Pd nanowires which respond to H2 with an increase in resistance have been achieved through various nanofabrication techniques and have been systematically investigated.8,9,22-24,26-27 Both experimental and simulation results show that their H2 sensing ability increases and their response time decreases when the sensors' transverse dimensions shrink. The results clearly demonstrate that Pd nanowires can be excellent sensing elements for highly sensitive and fast acting H2 sensors. The utilization of single palladium nanowires, however, faces challenges in nanofabrication, manipulation, and achieving ultrasmall transverse dimensions.
Scientists have developed/utilized various approaches to fabricate single Pd nanowires: (1) electrodepositing Pd at the step-edges on graphite; (2) electrodepositing Pd into nanochannels of porous membranes, for example anodic aluminum oxide, and (3) patterning Pd films via electron-beam (e-beam) lithography or deposition and etching under angles (DEA) methods. The last approach is costly because nano- (for example e-beam lithography tool) and microfabrication machines are expensive. It is difficult to achieve single nanowires with diameters smaller than 20 nm through these physical patterning techniques. In the first approach, it is inconvenient to reproducibly and massively fabricate single Pd nanowires by electrodepositing them on step-edges of graphite substrates. Furthermore, the nanowires grown on conducting graphite need to be relocated to an insulating substrate. Electrodepositing Pd into nanochannels of porous membranes is a convenient way to obtain large quantities of Pd nanowires. However, the problem with this method is making electrical contacts to individual nanowires, which typically requires the use of photo- or e-beam lithography and subsequent film deposition, resulting in a tedious fabrication process. Moreover, the surfaces of these nanowires can be contaminated during the process of dissolving the porous membranes to release the nanowires, degrading their gas sensing performance.